“Is” is only what the mind remembers …
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The "Horizon" theory (related to "light cones") says that the ability for an inertial object to observe a reference object is limited to the medium of observation. In other words, an observation must be transmitted by information and information carried by light, sound, x-rays, gamma rays, etc. can only transmit at a limited speed. This becomes a problem of physics which hinges on the entire concept of reality. Our entire reality is limited to only that which can be observed through instruments which in turn can only be interpreted by us if something traveling the speed of light or slower can transmit it.
The "Horizon" theory goes like this: If the "cause" of observation for 3 dimensional space is limited to a speed of approximately the speed of light, then some pieces of matter will never be able to be "seen" (or detected) by the others which have moved in an opposite direction where the total speed is faster than light - because those particles traveling at one half the speed of light in one direction trying to observe another which has traveled half the speed of light in the opposite direction would require that the relative speed of one particle in reference to the other would be the speed of light or greater and there is no medium of information transfer in which the one could possibly detect the other. In other words, there is a problem of adding velocities - they should be moving the speed of light relative to each other.
Another Horizon problem is history. Any occurrence that happens at a time in history longer ago than an information medium could carry, creates an occurrence outside of the observer’s “Horizon.” If two pieces of matter are older and further apart in time than an observation medium like light or other rays could transmit information, there is no way for that occurrence to be observed or recorded by another and hence no way for them to exist in the same “reality.”
Now we are not saying that both occurrences could not exist in some reality — we are just saying that they cannot be in the same physical reality, because there is no physical means for one to detect the other. Even the "Lorentz" time constriction or dilation explanation cannot help us out here, because we are talking about a beginning point where the only reference is the spot from which the movement or occurrence began.
But if velocities of two particles each traveling the speed of light from one reference point (the location of the big bang for example) in two different directions, the relative speed between the two particles cannot be greater than the speed of light, or the speed of light restriction to approximately 299,792,458 meters per second has been violated - as to the two particles observing each other. Ergo, the two particles cannot observe each other, because they are not within each other’s "Horizon." The two particles have traveled away from each other faster than any information carrier, like light, sound, or electrical impulse, for example, could possibly "transmit." Therefore, they do not exist in each other’s reality, yet they both exist in the reality relative to the big bang.
This becomes a physics "problem" in that one can neither enter a "speed of light" number into a calculation for the "size of the universe" nor enter a "time of creation" of the universe to arrive at a limit for reality. It is a question of "being" in the abstract. Reality is an unsolvable problem because "now" or "is" does not actually exist. Time's arrow will not allow for it. "Is" is only what the mind remembers it to be.
What really is Relative Velocity (Vi - Vr)?
You might just as well be asking what is a “Full Stop”? For example, naturally when on the Ocean, an earthbound sea vessel will never come to a complete stop. The ocean currents and waves are always fluctuating and the ship does not fully stop (in relation to the earth). Until one faithful day long in the future when that ship either sinks to the bottom of the ocean or is put into dry dock, the ship will always be moving (even if ever so slightly on a relative calm sea) in relation to dry land. Even when the ship drops anchor, the ship continues to rock and rotate. In nautical terms, “Full Stop” simply means to “stop the propeller.” The ship may continue to be carried forward by momentum and slowed by friction, but eventually (it can take hours) the ocean currents themselves take hold of the ship’s motion relative to the dry land.
Likewise in the formula E = MC2, it is not the true statement of Einstein’s equation in his theory of special relativity that Energy equals the Mass of a particle times the speed of light squared. In fact, taking out the relative velocities from which the Energy is being measured (derived or observed) in relation to the mass and the speed of light, is the theory of special “anti-relativity.”
“Relativity” is saying that light speed is an arbitrary limit on velocity which defines our ability to observe “reality.” Much like the earth defines relative motion for a sea vessel— light defines the ability for one particle to detect (or to be detected by) another particle. At any moment-location in the space-time continuum, you may simply assume that somewhere there is a location where an actual “Full Stop” might occur and measure the relative speed of light from that position. Then you go to another position with a relative velocity from the first position, let’s say 20,000 meters per second moving away from the first position. Once again, you measure the speed of light, and you get the same result. The light speed is not relative to the first assumed “Full Stop” position. So, why then would you remove the (Vi - Vr) from the equation? The non-virtual form of the Einstein equation simply leaves relative velocity in the equation so that the true decaying nature of reality can be better understood.
The broader view of Einstein’s equation (including the inertial and reference velocities as shown in the equation on these Non-virtual pages) helps us understand that there is a yet to be discovered place in the galaxies where a true “Full Stop” may occur. The Galaxies are spinning, the stars or suns are moving, and the planets are rotating about those stars or suns. Yet, relative motion will always exist. When a space vessel accelerates in one direction or even spins in relation to its present velocity, the forces of acceleration will be felt by that ship and/or its passengers (if any). While an accelerometer can detect the forces of relative motion, only an observation medium that allows for the motion in “reality” to exist, allows for an inertial system to detect a reference system. This simple act of detecting another system lies at the heart of the macro-level quantum physical collapse of probabilities and the subsequent entanglement of the inertial system from which the reference system is observable.